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[email protected] CONFErENCE EdITION @CharlotteMBird BRAINTEASERS: EVOLUTION AND VARIATION IN CYNODONT ENDOCRANIAL ANATOMY

Charlotte M. Bird1,2, Stephan Lautenschlager1, Paul M. Barrett2 PEERING INSIDE THE SKULL 1 School of Geography, Earth and Environmental Sciences, University of Birmingham, UK 2 Department of Earth Sciences, Natural History Museum, London, UK New anatomical descriptions of the Thrinaxodon brain and inner ear highlight structurally similar endocranial anatomy across all cynodonts studied, with a foramen magnum, cerebellum, paraflocculi, cerebral hemispheres and olfactory bulbs (figure 1).

Cloud Compare analyses of the Thrinaxodon models (figure 3) shows millimeter scale size variations amongst the reconstructed elements, alongside slight shape changes in the olfactory region (figure 4). Figure 3. Digital endocast summary of the three studied With the three Thrinaxodon Thrinaxodon liorhinus specimens. specimens all being at various stages of adulthood, this indicates the potential for identifying intraspecific variation. Cerebellum However, the variation is not Cerebral hemispheres significant enough to make a Foramen definitive conclusion as to magnum Figure 1. CT scan of a Thrinaxodon skull (length 74mm) whether the changes result from and associated reconstructed endocranial anatomy subjectivity during the model (brain – blue, length 35mm; inner ear – yellow; neurovascular structures – red). making process or are natural Figure 4. CloudCompare analyses reveal intraspecific variations. It is thus necessary to differences in the olfactory bulbs (blue) for specimens UMZC produce further models for this T815 and NHM 511 respectively.

Olfactory bulbs species for a plethora of skull sizes to assess the presence of Paraflocculi ontogenetic variation.

Calculated encephalisation quotients for Thrinaxodon vary between 0.15 and 0.2, consistent Using 3D reconstruction techniques to investigate with other, more derived cynodonts (figure 5). However, a cognitive change accompanies the origination of Mammaliaformes, with EQ values between 0.5 and 1.09 for Monodelphis, intraspecific and ontogenetic variation within suggesting a more primitive cognitive ability for Thrinaxodon, despite a similar body size. endocranial structures, quantifying relative Regarding inner ear anatomy, Thrinaxodon possessed larger semi-circular canals than later cynodonts (for example, Brasilitherium). With these canals being responsible for coordination cognition, sensory capabilities and anatomical and agility (by detecting angular accelerations and decelerations in the head), it may be that evolution along the cynodont-mammal lineage. inner ear anatomy was a strong influence upon Thrinaxodon being able to hunt successfully. Moreover, calculation of a best hearing frequency between 1.87 and 2.41 kHz for Thrinaxodon is considerably more restricted than in Monodelphis domestica (8 to 64 kHz; Frost and Masterton 1994). Consequently, Thrinaxodon’s poor auditory acuity may have been compensated for by larger olfactory bulbs than in later cynodont genera, thus ONCE UPON A TIME... supporting a burrowing life mode and nocturnal hunting style.

Fossils had to be destroyed to make a model of the brain, but thankfully virtual palaeontology has come to the rescue, utilising non-destructive modeling techniques to restore soft tissues lost to time. Such methods are of particular interest for the understudied Cynodontia, a group of therapsids that survived the end-Permian extinction event to radiate during the Triassic, with later Mammalian origination.

Cynodont brains are yet to be comprehensively studied for morphological evolution and as such, no known attempts have been made to identify intraspecific (between members of a species), ontogenetic (life cycle) and sexually dimorphic variation. With Thrinaxodon liorhinus exhibiting the most abundant cynodont fossil record, it thus facilitates study of brain, inner ear and neurovascular anatomy (figure 1) to identify these changes and permits further understanding of the evolution of sensory capabilities and subsequent behavioural patterns.

Moreover, the methodology provides opportunity to discover biases in digital reconstruction techniques resulting from the software used and the subjectivity of the model maker.

PAINTING THE PAST

High resolution CT scans of cynodont skulls facilitate 3D endocranial reconstruction within Figure 5. The evolutionary development of mammalian brain morphology, highlighting cognitive compartmentalisation from the late Avizo (and comparative software, including SPIERS, VG Studio and Mimics). Linear and Permian onwards. volumetric analyses within Avizo and CloudCompare assess size and shape changes for individual parts of the brain and inner ear, with calculation of encephalisation quotients (a measure of relative cognitive ability) and best hearing frequencies possible for comparison with more recent cynodont genera and the modern Monodelphis domestica (figure 2). CONCLUSIONS REFERENCES

Frost, S.B. & Masterton, R.B. 1994. Hearing in primitive The lack of basicranial bones in mammals: Monodelphis domestica and Marmosa elegans. the braincase and subjectivity Hearing Research, 76, 67–72. during the modelling process Brasilitherium riograndensis Macrini, T.E., Rowe, T. & VandeBerg, J.L. 2007. Cranial Thrinaxodon liorhinus UFRGS-PV-1043-T pose some of the greatest Endocasts from a Growth Series of Monodelphis domestica NHMUK PV R 511 (Rodrigues et al. 2014) (Didelphidae, Marsupialia): A Study of Individual and Riograndia guaibensis challenges to studying cynodont UFRGS-PV-596-T Ontogenetic Variation. Journal of Morphology, 268, 844–865. (Rodrigues et al. 2018) endocranial anatomy. Yet analysing a larger dataset of Rodrigues, P.G., Ruf, I. & Schultz, C.L. 2014. Study of a digital cranial endocast of the non-mammaliaform cynodont skulls over a plethora of size Brasilitherium riograndensis (Later Triassic, Brazil) and its Thrinaxodon liorhinus Monodelphis domestica ranges and temporal relevance to the evolution of the mammalian brain. UMZC T815 Figure 2. Sample of the TMM M-7599 Paläontologische Zeitschrift, 88, 329–352. (Macrini et al. 2007) distributions will provide new species studied insights into the evolution of the Rodrigues, P.G., Martinelli, A.G., Schultz, C.L., Corfe, I.J., Gill, mammalian brain from its P.G, Soares, M.B. et al. 2018. Digital cranial endocast of ancestral forebears. Riograndia guaibensis (Late Triassic, Brazil) sheds light on the evolution of the brain in non-mammalian cynodonts, Historical Biology, 1–18.